On
Earth, water occurs in three physical states: solid (ice), liquid, and
gas (vapor). Its most common state is liquid as oceans,
rivers, lakes and ponds. It is also present in much smaller
quantities in organisms and soil. The water cycle operates within a rigorous
balance, but is related to many factors associated with climate, vegetation
cover, wind conditions, and the geographical location of large bodies
of water such as rivers and lakes.

Water
is an essential component of metabolic activities and acts as a primary
transportation medium for both organic and inorganic substances.
In plants it is fundamental to structural maintenance and in animals it
is important to thermal regulation.

Water
vapor is found in the atmosphere and is created by evaporation, mainly
from bodies of liquid water, but also from the soil and by plant transpiration;
and to a minor extent through respiration, excretion and perspiration
processes in animals. After the organism's death, water comes
back to the environment through decomposers' action. The energy
source for this is mainly solar radiation. Tree cover - such as
the rainforest canopy - and vegetation (including seaweed) can play a
major part in regulating rates of evaporation. Cloud cover is created
by the evaporation process as a result of phase changes in the water,
due to alterations in pressure and temperature.

Plants
absorb ground water through their roots and the consumers (ie animals)
absorb it directly by drinking, or indirectly through the consumption
of plants and other animals.